Method and arrangement for dynamic signalling

ABSTRACT

The present invention relates to a method and an arrangement for dynamic signalling in a network, such as a Dynamic Synchronous Transfer Mode (DTM) network. The network&#39;s bandwidth is divided into cycles, which in turn are divided into control slots for control signalling and data slots for transferring data. Preferably, each or certain nodes have a node controller, which control the access to slots. According to the invention data slots are converted into control slots, called dynamic control slots, or control slots are converted into data slots in order to change the signalling capacity of a node. The dynamic control slots may be used for point-to-point, multicast or broadcast signalling.

TECHNICAL FIELD OF INVENTION

The present invention relates to a method and an arrangement for dynamicsignalling in a time multiplexed network, such as a Dynamic SynchronousTransfer Mode (DTM) network.

DESCRIPTION OF RELATED ART

DTM is a broadband network architecture (see e.g. "The DTM GigabitNetwork" by Christer Bohm, Per Lindgren, Lars Ramfelt and Peter Sjodin,published in Journal of High Speed Networks, Vol. 3, No. 2, pp. 109-126,1994, and "Multi-gigabit networking based on DTM" by Lars Gauffin, LarsHakansson and Bjorn Pehrson, published in Computer Networks and ISDNSystems, Vol. 24, No. 2, pp. 119-139, April, 1992).

DTM is a circuit switched network intended to be used in public networksas well as in local area networks (LAN's). It uses channels as thecommunication abstraction. The channels differ from telephony circuitsin various ways. First, the establishment delay is short so thatresources can be allocated/reallocated dynamically as fast as userrequirements change. Second, they are simple and so minimise overheadwhen the communication is unidirectional. Third, they offer multiplebit-rates to support large variations in user capacity requirements.Finally, they are multicast, allowing several destinations.

DTM channels share many beneficial properties with circuits. There is notransfer of control information after channel establishment, resultingin very high utilisation of network resources for large data transfers.Support of real-time traffic is natural; there is no need for policing,congestion control or flow control within the network. The controlinformation is separated from data information, making the data flowswiftly through the network without being manipulated in networkswitches. The switching delay is negligible (i.e. less than 125 μs) andthere is no potential for data loss because of buffer overflow as in ATM(ATM--Asynchronous Transfer Mode). Bit error rates depend on theunderlying media technologies, and switches are simple and fast due tostrict reservation of resources at channel setup. DTM can show goodperformance in areas where traditional circuit-switched networks fallshort: dynamic bandwidth allocation, channel establishment delay, and asshared media networks.

The basic topology of a DTM network preferably comprises a bus with twounidirectional optical fibers connecting all nodes, but can also berealised by any other kind of structure, e.g. a ring or hub structure.The DTM medium access protocol is a time-division multiplexing scheme.The bandwidth of the bus is divided into 125 μs cycles, which in turnare divided into 64-bit time slots. The number of slots in a cycle thusdepends on the network's bit-rate.

The time slots are divided into two groups, control slots and dataslots. Control slots are used to carry messages for the network'sinternal operation. The data slots are used to transfer user data.

Generally, in each network node there is a node controller, whichpreferably controls the access to data slots and performs networkmanagement operations.

Control slots are used exclusively for messages between nodecontrollers. Each node controller preferably has write permission to atleast one control slot in each cycle that it uses to broadcast controlmessages to other nodes. Here, broadcast refers to sending informationto all downstream nodes on a bus as the transmission medium isunidirectional. Since write access to control slots is exclusive, thenode controller has always access to its control slots regardless ofother nodes and network load.

However, if there are many nodes on the same bus, the signallingoverhead represented by the control slots may constitute a large part ofthe total capacity. It is therefore desirable to keep the controlsignalling capacity as low as possible. On the other hand, the controlsignalling capacity determines much of the performance of the network,both for access delay and utilisation. During periods of much controlsignalling, it is consequently advantageous to have a high controlsignalling capacity. Also, the control signalling demands varysignificantly between different nodes and may also vary in time for onesingle node.

SUMMARY OF THE INVENTION

The object of the invention is to optimise the use of control slots anddata slots in a network, such as a DTM network, whose bandwidth isdivided into cycles, which in turn are divided into control slots forcontrol signalling and data slots for transferring user data. It isdesirable to have a high bandwidth for data communication and a highcapacity for control signalling when requested.

This is accomplished with a method, a controller, a node and a networkaccording to the claims below. Hence, according to the invention, thereare provided means for dynamical adjustment of the control signallingcapacity of a node depending on its control signaling needs.

In the dynamic signalling concept, the signalling capacity of a node isincreased by the conversion of data slots into control slots, which maybe called dynamic control slots, and is decreased by the conversion ofcontrol slots, preferably said dynamic control slots, into data slots.

In a preferred embodiment, one slot per cycle or at least one slot pern'th cycle (n is preferably chosen between 1 and the number of nodes onthe DTM bus), called a static control slot, is left unconverted.

When data slots are converted into control slots related to a node, alsoall of the relevant downstream nodes may be informed about theconversion through status messages sent in the static control slot.

As a measure for deciding whether or not a node should convert dataslots to control slots or vice versa, the length of a queue for outgoingcontrol slots in the node may be used.

The converted slots can be used for either point-to-point, multicast orbroadcast signalling. This dynamic signalling concept can be combinedwith a slot reuse method to enable simultaneous control signalling anduser data transmission in the same slot over disjointed segments of abus or a ring.

Consequently, according to the invention, the problems mentioned aboveare avoided by dynamically optimising the control signalling bandwidthfor each node.

An advantage of the invention is that it provides a simple and easilyimplemented mechanism, which strongly improves the performance of thenetwork, especially at lower bit rates. If at least one slot per cycleis left unconverted as a static control slot, no hardware change to theoriginal prototype implementation is needed.

A further advantage of the invention is that by minimising the controlsignalling overhead, more nodes can be connected to each bus or ring.

Yet another advantage is that any kind of equipment can be connected tothe network independently of its signalling needs.

Still another advantage of the invention is that, when dynamic controlslots are used for signalling to one or several other nodes, the load onthe rest of the nodes does not increase.

Yet another advantage is that an improved bandwidth utilisation isachieved when combined with a slot reuse method.

Further aspects and features of the invention will be apparent from theaccompanying claims and the description below.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplifying embodiments of the invention will be described in greaterdetail below with reference to appended drawings, in which:

FIG. 1 schematically illustrates a dual-bus DTM network;

FIG. 2 schematically illustrates a DTM 125 μs cycle with data slots andcontrol slots;

FIG. 3 schematically illustrates n nodes sharing the same control slotbut on different cycles within the base frame according to theinvention;

FIG. 4 schematically illustrates different signalling channels, whichdefine different virtual networks according to the invention;

FIG. 5 schematically illustrates a point-to-point signalling channelaccording to the invention; and

FIGS. 6-9 schematically illustrate results from simulations wheredynamic signalling is included.

DETAILED DESCRIPTION OF THE INVENTION

First, the Dynamic Synchronous Transfer Mode Medium Access Control (DTMMAC) type of protocol will be described with reference to FIGS. 1 and 2.

An example of a basic topology of a DTM network is a bus with twounidirectional optical fibers connecting all nodes, as shown in FIG. 1.Several buses with different bit-rates may be connected to form anarbitrary multi-stage network. In the current prototype implementation,buses can be combined into a two-dimensional mesh. A node at thejunction of two buses can synchronously switch data slots between thetwo buses. This allows for efficient switching with constant delaythrough the node. The primary communication abstraction in DTM is amulti-rate, multicast channel.

The DTM medium access protocol is a time-division multiplexing scheme,as is shown in FIG. 2. The bandwidth of the bus is divided into 125 μscycles, which in turn are divided into 64-bit time slots (or slots forshort). The number of slots in a cycle thus depends on the network'sbit-rate; for instance, on a 6.4 Gbit/s network there are approximately12500 slots per cycle.

In the DTM protocol, control information is separated from data. Byusing this kind of out-of-band control signalling, the demand forprocessing incoming data at high speed decreases, i.e. messages thatcontain control information can directly be forwarded to the protocolprocessing unit in the receiving node without having to process incomingdata aimed for an end unit. To accomplish this, the slots are logicallydivided into two groups, control slots and data slots.

Control slots are used to carry messages for the network's internaloperation, such as messages for channel establishment, channel releaseand bandwidth reallocation. The data slots are used to transfer userdata and are not interpreted by intermediate network nodes. Intermediatenodes are nodes between the source node and the destination node ornodes.

In each network node there is preferably a node controller NC, whichcontrols the access to data slots by creating and terminating channelson demand from users. Also, the node controller NC performs networkmanagement operations, such as network start-up and error recovery, bothin response to user requests and in the background.

Control slots are used exclusively for control messages between nodecontrollers. Each node controller preferably has write permission to atleast one control slot in each cycle, which it uses to broadcast controlmessages downstream to other nodes. The write access to control slots isexclusive and hence, the node controller always has access to itscontrol slots regardless of other nodes and network load. However, iffor example a node is provided for read purposes only, it need not beneccesary to give the node write access to control slots.

According to an exemplifying scheme of operation, each node is initiallyassigned a predefined number (one or more) of control slots, calledstatic control slots, at start-up. If there is more than one controlslot available to a node, its control slots may be evenly spread overthe frame to get a low average access delay to control slots, since anode that shall transmit a control message must wait for a control slotto pass the node. With evenly distributed control slots the averagedelay will be smaller.

The signalling capacity of a node is determined by the number of controlslots it possesses, which normally is constant. This is, however, notalways an optimal choice. For instance, if there are many nodes on thesame bus, the control signalling overhead represented by the controlslots may constitute a large part of the total network capacity. It istherefore desirable to keep the control signalling capacity as low aspossible.

On the other hand, the control signalling capacity determines much ofthe performance of the network, both for access delay and utilisation.During periods of much control signalling, it is consequentlyadvantageous to have a high control signalling capacity. Also, thecontrol signalling demand varies significantly between the nodesdepending on the equipment attached thereto. The control signallingdemand may also for one node vary in time. For instance, during daytime, office equipment is active while most home equipment is inactive.During nights, the opposite is true.

According to the invention, it is possible in for example a DTM networkto dynamically change the control signalling capacity individually foreach node, i.e. it is possible to change the number of control slotsassigned to each node during network operation. This is referred to asdynamic signalling.

According to the invention, dynamic signalling provides means toincrease the control signalling capacity of a node through convertingdata slots to control slots and to decrease the control signallingcapacity of a node by converting control slots to data slots. The dataslots converted to control slots are called dynamic control slots.Preferably, but not neccessarily, the static control slots are alwaysleft unconverted.

The allocation is performed by allocating a data slot, either a dataslot already assigned to the node or for example a data slot borrowed orovertaken from another node, marking it as control slot and theninforming all the downstream nodes on the bus about the allocating,which is done through special status messages sent in the node'sexisting control slot(s). The downstream nodes can then start to readthe allocated control slot.

Outgoing control messages handled by a node are queued to be sent in acontrol slot. Control signalling channel overload is recognised by, e.g.the size of the queue for outgoing control slots. Consequently, when thequeue is long the control signalling capacity may be increased and whenthe queue is short the control signalling capacity may be decreased. Thenumber of slots to be converted may also be correlated to the queuelength. Requests for conversions may also originate from the equipmentattached to the node. It is also possible for example, to manuallyinitiate the conversion and decide how many slots that are to beconverted.

In the simplest implementation, the node to convert data slots selectsfree data slots that it has access to. In a more sophisticatedimplementation the node may request and convert slots that belong toanother node and that may be situated, within the cycle, far fromexisting control slots. This is done in order to decrease the channelsetup time due to lower waiting time for passing control slots.

This scheme has the disadvantage that, generally, all nodes must haveaccess to at least one static control slot per cycle. For attachingsimpler terminals to the network, one slot per cycle may introduceunnecessary overhead. For large networks with many nodes connected toeach bus or when the link capacity is low, this overhead can be veryhigh. To avoid this, a solution based on base frames has beenimplemented. Several cycles are then organised into base frames, eachconsisting of n cycles, as shown in FIG. 3. The value of n is notspecified, but should preferably be between 1 and the number of nodes onthe bus. Then, each node preferably has access to at least one controlslot per base frame and will thus have smaller control signallingcapacity and longer delay for accessing but, on the other hand, theoverhead is less. Dynamic control slots can then be allocated accordingto the invention as described above.

The slot(s) that a node possesses within the base frame is called itsbasic signalling channel (BSC). The n first nodes share one controlslot, the n next another, etc. They share the control slot in the sensethat their control slots have the same number in the cycle, but aresituated in different cycles within the base frame. The capacity of theBSC's decreases and the delay to access the BSC's increases when nincreases. A disadvantage with using base frames is that it requiresextra hardware support since nodes need to count the cycles in a baseframe. This scheme has similarities with the use of multiframes andsuperframes in a GSM-protocol (Group Switching Mobile protocol).

The information sent in control messages can be of many different types.Some information is specifically directed to a set of receivers whileother information is more general. For example, information for settingup a channel generally only concerns the sender, the receivers andintermediate nodes. This type of control information is normally mostefficiently supported by point-to-point or selected multicast channels,while more general information, such as status information or queriesfor mobile terminals, may be best supported by broadcast channels. It istherefore not reasonable to broadcast all control information to allhosts on a bus.

The dynamic signalling is taken one step further in the virtual networkconcept. This concept provides means to superimpose several logicalstructures on top of the underlying physical infrastructure. The networkcan then dynamically adapt its logical structures to for example trackchanging traffic conditions. Also, many different types of signallingneeds can be foreseen: signalling sent to a specific node for handling aconnection, e.g. in a client-server scenario, signalling to multiplenodes in a multicast scenario, e.g., to handle a teleconference, andbroadcast signalling that is typically used to spread statusinformation, e.g., routing or node state information. The virtualnetwork signalling concept supports these different alternatives in anefficient way.

Now referring to FIG. 4, to implement the virtual network concept, thechannel abstraction, i.e., point-to-point, multicast or broadcastchannels, that is used for transferring data in the DTM protocol, isalso applied to control signalling channels. The signalling messages arethus not broadcast as in the original DTM protocol, but are directed toone or several specific nodes. These nodes will then have their ownlogical control signalling channel and form a virtual network. Thecapacity of these signalling channels can be dynamically changedaccording to the demands of the nodes. Since the DTM channel concept isnot restricted to a single bus, a virtual network may span severalbuses.

A node creates (or joins) a virtual network by signalling to the nodeswith which it wants to communicate, preferably by using the basiccontrol signalling channel (BSC). The control slot specifies which dataslots will be used for signalling within the virtual network. Theincoming slots that belong to this signalling channel will be attachedto a local logical channel similar to the logical channels used for datain the node. A node may join several virtual networks and will thereforehave many different incoming and outgoing signalling channels that areassociated with different virtual networks, as shown in FIG. 4.

The virtual network concept further makes it possible to allocateresources strictly for the nodes attached to the virtual network. Thenodes that belong to this network may then reallocate the slots amongthemselves, according to the slot allocation algorithm used. It is thuspossible to use different slot allocation algorithms for different typesof traffic. The concept provides functionality for, e.g., groupcommunication.

Virtual networks have the largest benefits in access networks, wherenodes communicate sporadically. For instance, when a video monitor isswitched off or a workstation is idle, the need for control signallingis very small. In that case, the node may only use the BSC. When aworkstation is used, it often communicates with its server. Since thetraffic from computers usually is bursty, there is a need for morecontrol signalling capacity between the workstation and the server. Forthis sake, it may establish a point-to-point signalling channel to theserver, as shown in FIG. 5. During for example a teleconferencingsession, a virtual network may temporarily be created between the nodesparticipating in the conference. The signalling capacity is thusallocated according to the current demands of the involved nodes. Nodesnot attached to the virtual network are unaffected by the increase incontrol signalling.

This concept has similarities to the signalling virtual channels (SVC's)used in the Asynchronous Transfer Mode protocol (ATM). There is onemeta-signaling virtual channel (MSVC) per interface that is permanent.Also, there are several other types of signalling virtual channels thatare allocated between signalling end-points while they are active.

The dynamic signalling concept can be combined with a slot reuse methodto further improve the bandwidth utilisation. Nodes are connected by bussegments and signalling channels using static or dynamic control slotsfor point-to-point or multicast signalling and use only a subset of thesegments on the bus, the remaining segments being reserved but leftunused and thus wasting shared resources. A better alternative is to letthe signalling channels only reserve capacity on the network segmentsbetween the source and the destinations), and thus only data slots onthese segments are converted into control slots. The data slots on therest of the segments are left unconverted and hence other nodes areenabled to use these slots for data transmission or to convert them intocontrol slots for their signalling purposes. A single slot may in thiscase be used multiple times on the bus, both as a (dynamic) control slotand as a data slot. Slot reuse enables, consequently, simultaneoussignalling and data transmission in the same slot over disjointedsegments of the bus.

Results from simulations will be described in the following. Therequired signalling capacity depends on the size of the transfers.Obviously, for a given load, small transfers imply many channelcreations. Instead of having a large permanent signalling overhead anddimension the signalling capacity for the worst case situation, it isnatural to dynamically adjust the number of control slots according tothe current load. In the simulations there are an upper and lower boundfor the number of control slots per node. The upper bound is there sincewhen the amount of control slots increases over a certain level, the NCprocessor is the limiting factor in its capacity of sending controlmessages. The lower bound exists to avoid that a node reallocates toomany control slots, which will result in longer channel setup times dueto waiting time for passing control slots.

The number of control slots for the node is then adjusted by recordingthe access time to control slots. If the access time exceeds the time ofq frames, the signalling capacity is increased and if the access timedecreases below one frame, the signalling capacity is decreased.Practically, the access time may be measured as the queue length foroutgoing control messages. The dynamic signalling mechanism is mostimportant in an access network. Also, the affect is larger in networkswith many nodes. Therefore, the transmission capacity, for thissimulation, is low and the number of nodes is high. The number of slotsin the cycle is set to 1200, which corresponds to a link capasity of 611Gbit/s and the network has 40×40 nodes. Some of the nodes are defined ashot-spots, i.e., they need a much higher signal capacity then the restof the nodes. Status messages that announce allocation of control slotshave higher priority than other control messages.

In FIG. 6 and in FIG. 7, results are shown from the simulations in termsof throughput and channel setup versus offered load. The dynamic controlslot assignment is compared with the static case with the upper andlower bounds assigned. The lower bound is here set to 4 slots per cycleand the upper bound is set to 10 slots per cycle. The value of q is setto 2, 4, 6 and 8 frames, respectively.

For the upper bound the overhead from the control slots will constitutea large part of the total number of slots which will reduce thethroughput. However, the mean access time is low. For the lower boundthe queues start to grow for higher loads and thus the channel setuptime will increase and the throughput decrease. As can be seen from FIG.6, a slightly better performance is obtained when q=2. This since theresponse on increasing queue length is faster and thus the channel setuptime will be low. The mechanism shall cope with long term requirementsfor higher signalling. As can be seen in FIG. 7 the mechanism is madeactive for offered loads higher than approximately 20%.

In FIG. 8 and in FIG. 9, the number of control slots assigned to eachnode are shown at start-up after approximately 500 μs of operation. Ascan be seen in FIG. 9, the number of allocated control slots varies butmany hot spots can be recognised. In general, the nodes close to theedges of the network will not be assigned the same number of controlslots since upper bound limits the number of control slots for one bus.An improvement of the scheme would be to relate the upper and lowerbounds to the location on the bus or to the total number of controlslots for a node.

The method and arrangement for dynamic signalling according to theinvention adjust dynamically the control signalling bandwidth for eachnode to its needs. An advantage of the invention is that it is a simplemechanism, which strongly improves the performance of, for example, aDTM network. If at least one slot per cycle is left unconverted as astatic control slot, no hardware change to the prototype implementationis needed.

By minimising the signalling overhead according to the invention, morenodes may be connected to the bus. Any kind of equipment may beconnected to the network independently of its signalling needs. Combinedwith a slot reuse method, the bandwidth utilisation is further improved.

The network is not restricted to one dual-bus or several dual-buses, butcan be realised by other kinds of structures, e.g., ring or hubstructures with an arbitrary number of nodes. The transmission mediacan, besides optical fibers, be coaxial cables or any other highbandwidth transmission media. The bandwidth of the DTM dual bus in thepreferred embodiment is divided into 125 μs cycles, which in turn aredivided into 64-bit time slots. The invention is not restricted to DTMnetworks with these values, but can be used in networks with cycles andslots of arbitrary sizes.

What is claimed is:
 1. A method for dynamic signalling in a networkhaving a plurality of nodes and a bandwidth, said methodcomprising:dividing said bandwidth into a plurality of time-sequentialcycles; dividing said plurality of time-sequential cycles into controlslots for network control signalling and data slots for transferringdata; and converting data slots into control slots or converting controlslots into data slots in order to change the signaling capacity of anode.
 2. A method according to claim 1, wherein said data slotsconverted into control slots are called dynamic control slots, andwherein only said dynamic control slots may be converted into dataslots.
 3. A method according to claim 1, wherein said method isimplemented in a Dynamic Synchronous Transfer Mode (DTM) network.
 4. Amethod according to claim 1, wherein at least one slot per cycle, calledstatic control slot, is left unconverted when converting control slotsinto data slots related to said node.
 5. A method according to claim 1,wherein at least one slot, called static control slot, every n'th cycleis left unconverted when converting control slots into data slotsrelated to said node, the value of n being preferably selected as aninteger value from one to the total number of nodes in said network. 6.A method according to claim 5, wherein said cycles are counted in orderto localise said static control slot related to said node.
 7. A methodaccording to claim 5, wherein the respective static control slotsrelated to different nodes are arranged in the same position within thecycle but in different cycles within a frame.
 8. A method according toclaim 1, further comprising informing downstream nodes about saidconversion related to said node.
 9. A method according to claim 1,further comprising using the length of a queue for outgoing controlslots related to said node, said queue representing the control slotdemand, as a measure for deciding whether or not data slots shall beconverted into control slots, related to said node, or vice versa.
 10. Amethod according to claim 1, further comprising using the length of aqueue for outgoing control slots related to said node, said queuerepresenting the control slot demand, as a measure for deciding how manydata slots that shall be converted into control slots, related to saidnode, or vice versa.
 11. A method according to claim 1, furthercomprising selecting a data slot that said node has access to as theslot to be converted into a dynamic control slot related to said node.12. A method according claim 1, further comprising selecting a data slotthat a second node has access to as the slot to be converted into adynamic control slot related to a first node.
 13. A method according toclaim 1, further comprising using said data slots converted into controlslots for point-to-point control signalling.
 14. A method according toclaim 1, further comprising using said data slots converted into controlslots for multicast control signalling.
 15. A method according to claim1, further comprising using said data slots converted into control slotsfor control signalling between at least two nodes on a common logicalcontrol signalling channel thus forming a virtual network.
 16. A methodaccording to claim 15, further comprising using said dynamic controlslots for allocating bandwidth to nodes attached to the virtual network.17. A method according to claim 15, further comprising connecting nodeson different buses to the same virtual network.
 18. A method accordingto claim 15, further comprising connecting a node to a virtual networkwhile being connected to at least one other virtual network.
 19. Amethod according to claim 1, further comprising said method beingcombined with a slot reuse method in such a way that data slots areconverted into control slots, or vice versa, only on segments connectingcommunicating nodes, a segment being the part of the transmission mediumthat connects a node to another node, leaving the same slots on othersegments of the network unconverted, and thus enabling other nodes touse these slots for data transmission or to convert them into controlslots or vise versa.
 20. A controller for a network containing at leasttwo nodes, the network having a bandwidth divided into cycles, eachcycle containing a plurality of slots, the controller comprising:meansfor dividing the plurality of slots into control slots for controlsignalling and data slots for transferring data, and means forconverting data slots into control slots and/or converting control slotsinto data slots in order to change the signalling capacity of a node.21. A controller according to claim 20, wherein said controller islocated in a node of a Dynamic Synchronous Transfer Mode (DTM network.22. A controller according to claim 20, wherein said controller isarranged to leave at least one slot per cycle or one slot per n'thcycle, called static control slot, unconverted when converting controlslots into data slots related to said node.
 23. A node in a network,said node comprising:a bandwidth divided into a plurality of sequentialcycles, each cycle divided into control slots for control signalling anddata slots for transferring data; and a node controller arranged toconvert data slots into control slots and/or to convert control slotsinto data slots in order to change the signalling capacity of said node.24. A network comprising:a bandwidth divided into a plurality ofsequential cycles, each cycle divided into control slots for controlsignalling and data slots for transferring data; and a plurality ofnodes, wherein at least one of the nodes comprises a node controllerarranged to convert data slots into control slots and/or to convertcontrol slots into data slots in order to change the signalling capacityof said node based on the queue length of control signals to be placedin outgoing control slots.
 25. A method according to claim 2, whereinsaid method is implemented in a Dynamic Synchronous Transfer Mode (DTM)network.
 26. A method according to claim 2, wherein at least one slotper cycle, called static control slot, is left unconverted whenconverting control slots into data slots related to said node.
 27. Amethod according to claim 3, wherein at least one slot per cycle, calledstatic control slot, is left unconverted when converting control slotsinto data slots related to said node.
 28. A method according to claim 2,wherein at least one slot, called static control slot, every n'th cycleis left unconverted when converting control slots into data slotsrelated to said node, the value of n being preferably selected as aninteger value from one to the total number of nodes in said network. 29.A method according to claim 3, wherein at least one slot, called staticcontrol slot, every n'th cycle is left unconverted when convertingcontrol slots into data slots related to said node, the value of n beingpreferably selected as an integer value from one to the total number ofnodes in said network.
 30. A method according to claim 6, wherein therespective static control slot related to different nodes are arrangedin the same position within the cycle but in different cycles within aframe.
 31. A method according to claim 16, further comprising connectingnodes on different buses to the same virtual network.
 32. A methodaccording to claim 16, further comprising connecting a node to a virtualnetwork while being connected to at least one other virtual network. 33.A method according to claim 17, further comprising connecting a node toa virtual network while being connected to at least one other virtualnetwork.